As a method for continously producing nitrogen-enriched gas from mixture gas such as air mainly containing oxygen and nitrogen, it is well known to repetitively perform adsorption and regeneration (desorption) with the use of an apparatus which comprises a plurality of adsorbers packed with carbon molecular sieve (CMS) as an adsorbent. This method utilizes the fact that the oxygen adsorbing ability of CMS varies with pressure.
In general, the oxygen adsorbing ability of CMS increases with increasing pressure. If air is passed through a CMS packed adsorber under high pressure, oxygen contained in air is adsorbed by the CMS to produce gas with a high nitrogen concentration. Conversely, if the adsorber is evacuated under atmospheric pressure or under vacuum to cause a pressure drop in the adsorber, oxygen is desorbed from the CMS, thereby regenerating the CMS.
In a multi-tower PEA method for separating nitrogen gas, one adsorber undergoes adsorption for producing nitrogen-enriched gas while another adsorber undergoes regeneration. Therefore, by alternately repeating these steps, it is possible to realize continuity in nitrogen-enriched gas production because, at all times, either one of the adsorbers produces nitrogen-enriched gas.
A nitrogen gas separating method based on PEA is advantageous for its capability of producing nitrogen-enriched gas conveniently at a relatively low cost. However, such a method is disadvantageous for its difficulty of increasing the purity of nitrogen gas, as compared with a method wherein liquefied nitrogen is vaporized to produce nitrogen gas for industrial applications. Various efforts have been hitherto made to enhance the nitrogen gas purity, but failed to provide satisfactory results. In particular, when using a simple apparatus which performs regeneration under atmospheric pressure, the achievable product nitrogen purity has been limited to 99.9%.
For example, Japanese Patent Publication No. 5-32087 proposes a nitrogen gas separating method which increases the nitrogen purity of product gas to 99.9% by PSA wherein regeneration is performed under atmospheric pressure. A first feature of this method resides in that high-purity product nitrogen gas is passed through an adsorber via its outlet while it performs atmospheric pressure regeneration. A second feature resides in that, after a so-called pressure equalization step, high-purity nitrogen gas from a product receiver is made to reversely flow into an adsorber for advance pressurization before starting adsorption.
The first feature described above, wherein high-purity nitrogen gas is passed through an adsorber undergoing regeneration, is conventionally well known as a measure for increasing the regeneration efficiency of CMS. The second feature described above is conventionally well known as a measure for increasing the purity of product gas in case vacuum is applied for regeneration where no rinsing is available. The method proposed in the above Japanese publication relies on the combination of these known features for additionally increasing nitrogen purity in PSA nitrogen gas separation. As previously described, however, the purity of product nitrogen is still limited to 99.9% even if the method disclosed in the above Japanese publication is employed.
It is, therefore, an object of the present invention to provide a method and apparatus for separating nitrogen gas which is capable of enhancing the nitrogen purity to 99.999% by relatively convenient PSA wherein regeneration is performed under atmospheric pressure.